High density recording has been in strong demand as a magnetic recording medium. In order to achieve high density recording, it is necessary to increase reproducing output, to decrease noise, or to increase C/N (ratio of carrier (C) to noise (N)) by both increased reproducing output and decreased noise.
In a coating type of magnetic recording medium which has conventionally been widely used, magnetic recording-reproducing characteristics of high density and high C/N are attained by changing the magnetic material to be used, for example, from iron oxide powder to metal powder, and also by improving the magnetic characteristics of the material or by decreasing the size of magnetic particles. However, a coating type magnetic recording medium is limited in terms of further improvement in high density recording and, therefore, as a new material for attaining high density recording, a metal thin film type of medium is now being developed.
Various methods are known for the formation of a metal magnetic thin film, including a vacuum film forming method in which a ferromagnetic metal is provided in a vacuum in the form of a thin film on a substrate, such as a polymer film, by a method, such as sputtering, ion plating or electron beam vapor deposition, or a plating method in which a thin film is formed on a substrate in an aqueous solution. Some magnetic tape media produced by the electron beam vapor deposition method are now put into practical use as magnetic tapes for a 8 mm video system, i.e., so-called Hi-8ME.
In recent years, an increase in recording density has been in strong demand, and the demand for high volume recording has also increased. In order to comply with this demand, an improvement in the characteristics of the metal magnetic thin film type of magnetic recording medium is now under investigation.
In order to improve these characteristics of the metal thin film type of recording medium, such as that produced by the electron beam vapor deposition method using Co--Ni, the following have been made:
(1) Improvement of the structure of particles in the magnetic film, through the increase of coercive force and reduction of noise by introducing oxygen at the time of vacuum vapor deposition.
(2) Reduction of noise by employing a multi-layer structure in a plurality of vapor deposited films, and improvement of recording-reproducing characteristics by improving magnetic characteristics of the magnetic film (including the provision of a non-magnetic intermediate layer).
(3) Removal of direction dependency of recording-reproducing characteristics by controlling the direction of growth of magnetic particles in an oblique vapor deposited film of multi-layer structure.
(4) Increase of durability by providing a non-magnetic intermediate layer between magnetic films in a multi-layer structure of oblique vapor-deposited films.
In accordance with the above methods, however, it has been impossible to increase the reproducing output and C/N over a broad recording signal band. In other words, with the metal thin film type of magnetic medium, a high reproducing output can be obtained in a region of shorter recording wave length, but in a region of longer recording wave length, the reproducing output is decreased compared to the coating type of recording medium.
In order to increase the output in the shorter recording wavelength region to attain high density recording, decreasing the thickness of the magnetic layer and increasing the coercive force and so forth are effective techniques. However, when these techniques are employed, the reproducing output in the longer recording wavelength region decreases to a greater extent. On the other hand, if the thickness of the magnetic layer is increased, the output in the shorter recording wavelength region is decreased. This has not been possible to attain along with high density recording.
In recent years, however, a medium for high density recording of high vision signals or digital signals of high transfer rate has been in increasing demand. For high quality and high density recording of the above signals, there is a need for a recording medium which permits one to obtain a high reproducing output over a broad frequency region, i.e., a broad recording wavelength region. However, no magnetic recording medium complying with these requirements has been obtained so far.